Process for impregnating wood and composition therefor



Patented Dec. 7, i948 PROCESS FOR. IMPREGNATING WOOD AND COMPOSITIONTHEREFOR William Karl Loughborough, Madison, Wis., as-

signor to United States of America as represented by the Secretary ofAgriculture No Drawing. Application March 9, 1945, Serial No. 581,821

6 Claims.

1 This application is made under the act of March 3, 1883, as amended bythe act of April 30, 1928, and the invention herein described andclaimed, if patented, may be manufactured and used by or for theGovernment of the United States of America for governmental purposeswithout the payment to me of any royalty thereon.

This is a contlnuation-in-part of my copend-' ing application, SerialNo. 462,191, filed October 15, 1942, now Patent No. 2,442,897, which, inturn,

is a division of my application, Serial No. 383,798,

Other objects of my invention are to permit,

for various woods, faster drying than can be obtained with ordinarydrying or chemical seasoning methods or to permit less exacting controlof humidity conditions than is required in other seasoning processes.Further objects are to provide a chemical seasoning agent that does notcorrode metal equipment used in the drying process or metal deviceswhich might be used with the dried wood, which will not materiallyinjure the inherent electrical properties of the wood, which does notdiscolor wood, and which protects the wood during seasoning, and, to acertain extent, during use subsequent to drying, against fungal stain,decay, insect damage, and fire.

Still further objects of my invention are to provide a chemicalseasoning process which will yield a product that is non-hygroscopic andwhich has permanentantishrink properties and increased strength acrossthe grain.

The first step in the process of chemical seasoning is the depositionwithin the outer layers of water-swollen wood of one or more hygroscopicand antishrink chemical agents. The expressions outer layers" and "outerzone as used hereinafter refer to that portion of the wood be- (Grantedunder the Act of amended April 30, 1928;

March 3, 1883, as 370 0. G. 757) tween the surface and a depth of aboutdistance from the wide surfaces to the midway between the wide surfaces.

The first step is usually accomplished by soaking the green wood in acomparatively concentrated solution of the chemical agent. Although thewater vapor pressure of such a solution is considerably lower than thatof the water in the wood and some is removed from the wood during thesoaking, the wood is usually taken from the solution before much dryinghas occurred, and most of the drying is accomplished by exposing thewood to a drying atmosphere.

When the chemical agents are absorbed, the relative vapor pressure ofthe water in the treated portion of the wood is lowered, and suchportion is given water retention properties not possessed by untreatedwood. The process reverses the normal stresses that develop in wood asit dries by causing the wood in the outer layers to retain moistureabove its fiber saturation point, and thus it remains in a turgidcondition even though a normal drying gradient is set up in the interiorof the wood. The chemical agent also diffuses into the cell walls of thewood where it prevents the wood from shrinking as much as it would if nochemical were present.

Heretofore, simple organic or inorganic compounds, such as sodiumchloride, monoammonium the plane phosphate, urea, diethylene glycol,glycerine, in-

vert sugar or mixtures of such compounds have been employed in chemicalseasoning processes. However, many of them, such as urea, do not haveadequate hygroscopic and antishrink effect during the drying periodadequately to decrease checking and honeycombing of thick pieces ofrefractory woods, such as oak. Those that are most eifective forreducing degradation of refractory woods have other disadvantages. Forexample, sodium chloride is so corrosive that it cannot be used in akiln containing any ferrous metal fixtures, and the presence of thionizable salt in the dried wood lowers its electrical resistance.Diethylene glycol, glycerine, and similar compounds are too expensivefor general use. Urea alone or in combination with chemicals such asinvert sugar, corn sugar, or molasses discolor wood. Compounds that tendto maintain their hygroscopic properties after the wood ha been driedresult in the treated wood having a tendency to absorb more moisturethan untreated wood when exposed to high relative humidities and tocondense moisture on its surface when exposed, even for short periods,to very high relativ humidities. Furthermore, chemical seasoning agentsof the prior art require a high degree of precision in the control ofkiln conditions. For example, when the humidity becomes appreciablyhigher than the relative humidity in equilibrium with a saturatedsolution of chemical seasoning agents of the prior art, excessivemoisture is absorbed by and condenses on the wood. It exposure to thishumidity is prolonged, the water will drip from the lumber, leaching outthe protective chemical agent. On the other hand, if the relativehumidity becomes appreciably lower than the relative humidity inequilibrium with a saturated solution of the chemical seasoning agent,the agent will not be effective in maintaining sumcient moisture in thewood to prevent checking, especially if excessive humidity at someprevious time has leached out part of the chemical.

My invention obviates to a large extent the deiiciencies or the priorart pointed out above by utilizing essentially monomeric water-solubleresin-forming compounds in the chemical seasoning process. The essentialattributes of such compounds that are efiective in carrying out theobjects and purposes of my invention are that their molecules havecomparatively small size and a structure such as to permit their readydiffusion into the free and hygroscopic moisture of the wood. Also,these compounds are sufficiently effective in lowering the relativevapor pressure of the water in the wood, and they eventually polymerizein the fine swelling structure of the wood during the latter stages ofthe drying process, forming a resin that has no appreciable hydroscopicproperties.

Buffers or other modifying agents may be used to assist in maintainingthe resin-forming compounds in a diifusible state and in regulating theformation of the non-hygroscopic resin during the latter stages of thedrying. Special heating or conditioning periods or secondary chemicaltreatments at the end of the drying may also be employed to complete thesetting of the resin if required in any particular case. Other chemicalagents may be added to the treating solution to obtain increasedprotection against stain, decay, insects, and fire.

Buffered urea-aldehyde solutions, described in Patent No. 2,313,953 andin my copening patent application, Serial No. 462,191, filed October 15,1942, are especially useful in the practice of my present invention.

It is essential that the chemical agent be confined largely to the outerlayers of the wood.. For this purpose, I have found it satisfactory tosoak the unseasoned wood in an aqueous solution of the chemical agent 3to 4 days per each inch of thickness of the wood.

In order for the chemical seasoning agent to accomplish its purpose, itsconcentration in the outer layers of the Wood must be high, andaccordingly the concentration of the chemicals in the treating solutionmust be high. Solutions in which the combined weights of the urea andformaldehyde components make up 40 to 60 percent of the total weight arepreferred for this purpose.

Dimethylolurea may be substituted for the formaldehyde and a portion ofthe urea in preparing the buffered urea-aldehyde so ut o A thiourea maybe used as a substitute for or in admixture with urea in the bufferedurea-aldehyde solution. Other aldehydes, such as acetaldehyde, may alsobe used in the solution.

Examples of typical solution formulae which have been successfully usedin the chemical seasoning of oak in accordance with my invention are:

Dimethylolurea (89 percent pure, 11 percent water) 286 Urea 253 Althoughthe formulae given above, which produce solutions having, approximately,pH 10, are suitable for use with oak, which is a highly acid wood, otherwoods that are not so highly acid may require the omission of the sodiumhydroxide or the addition of glacial acetic acid to reduce the pH of thesolutions to the approximate range of 8 to 9. I have found such a pH tobe ideally suited for maintaining the resin-forming compounds in avirtually unpolymerized state so that the treating solution is availablefor a longer period of time and, more importantly, the compounds aremaintained in a diffusible state without losing their hygroscopicproperties during the early stages of the drying.

The buffered urea-aldehyde solutions described above tend to bleach thewood rather than to darken it as in the case of urea or urea incombination with compounds such as diethylene glycol, invert sugar, andmolasses. The ureaaldehyde resin present in the wood after drying doesnot lower the electrical resistance of the wood as do inorganic salts.After completion of the drying, the urea-aldehyde resin isnon-hygroscopic. I

Wood which has been dried with the aid of the resin-forming compounds isless hygroscopic than untreated wood and much less hygroscopic than woodtreated with simple hygroscopic chemical agents heretofore used in thechemical seasoning process. It absorbs less moisture from air which hasa. high relative humidity than untreated wood, and much less moisturethan, for example. wood treated with urea. Moisture does not condenseupon the surface of the resin-treated wood when it is exposed to veryhigh humidities. The urea-formaldehyde resin cannot be leached out ofthe wood after drying is completed. Thus, the antishrink effect which isobtained by the treatment of the wood with the buffered urea-aldehydesolution is permanent. The urea-formaldehyde treated wood is notaffected by stain or decay fungi when exposed to high humidities duringor after drying. The treatment also increases the fire resistance of thewood.

Using a buffered urea-aldehyde solution, a

any thickening agent satisfactory procedure comprises preliminarysoaking of the unseasoned wood in the solution for about 3 or 4 days perinch of thickness of the stock. The wood is then dried in a dryingchamber under controlled temperature conditions with humidity conditionsmaintained in the beginning at approximately the relative vapor pressureof air in equilibrium with the treating solution. The drying conditionsare then adjusted so as finally to dry the wood to the desired moisturecontent, which will usually be about 3 to 15 percent by weight but ofthickness of the stock, and then either drying the wood under air-dryingconditions until it is dried to the desired moisture content orpartially drying it under such conditions and then completing the dryinin a drying chamber under controlled conditions.

Although most satisfactory results are obtained when the chemical agentis applied by the soaking method, other means of application may also beused in the practice of my invention. For example, I have found itsuitable to apply the solupaste-treated wood solid, preferably under awaterproof cover, and leaving it so piled until the chemical agents havediffused into the wood. Stickers about inch thick are satisfactory inusing this method. The diifusion period is about the same as required inthe soaking method. After the difl'usion period, the treated wood isdried in the same manner as the soaked wood.

Another means of handling such paste-treated wood is to pile the wood onstickers within a drying chamber, then maintain the relative humidity ofthe air in the chamber at equilibrium with the relative vapor pressureof the chemical seasoning agent until the chemical compounds havediffused into the wood. The drying phase is then effected.

In carrying out the variations described above, which will increase theviscosity of the treating solution without adversely aiTecting itsproperties or the properties of the wood being treated can be used, suchas cornstarch, methylcellulose, bentonite, and methylcellulose incombination with bentonite. Any other hydrophilic thickening agent maybe used. The paste can be applied by dipping, brushing, spraying, orother means. The amount of chemical seasoning agent retained by thewaterswollen wood when the paste is applied by dipping, brushing,spraying, and so forth, may be controlled by varyin the amount ofthickening agent used and by other means, such as regulation oftemperature, and so forth. The amount of chemical seasoning agentrequired will depend upon the size and kind of wood being dried, but thepractical range is from about 40 to about 200 pounds of resin-formingcompounds per thousand board feet of wood.

Under some circumstances, and for some wood pieces, it may besatisfactory to impregnate the outer zones of the unseasoned wood withthe treating chemicals by means of a pressure or vacuum-pressuretreatment.

The following examples illustrate my invention in more detail and themanner in which it may be practiced:

Exauru 1 Soaking and kiln drying method Parts by weight Water 231 Sodiumhydroxide -r 6 Borax 39 Urea 380 Formaldehyde (37 percent solution) 344The first four ingredients listed were mixed together in the order shownwhile heating to 150 F. The last ingredient was added after the mixturewas cooled to approximately 100 F. The temperature of the solutionduring soaking was about F. 7

After the soaking period, the ends of the pieces were treated with amoisture-resistant coating to prevent moisture loss from the ends of thepieces and consequent end checking. The soaked and end-coated pieceswere piled on l-inch stickers in a dryin chamber along with 40 similarendcoated pieces which had been soaked 12 days in a saturated ureasolution at approximately 80 F. and 40 similar end-coated pieces whichhad not been given any chemical seasoning treatment. All these pieceswere then dried to a moisture content of approximately 12 percent, basedon the oven-dry weight of the wood, using the following drying schedule:

Temperature Moisture Relative content of Elapsed time, days Dry wethumidity, untreated bulb bulb percent specimens,

O o percent the influence of the The following table shows bufl'eredurea-aldehyde treatment:

.TABLE A When drying was completed, the urea-treated oak wasconsiderably darkened compared with the untreated controls, but thebuffered ureaaldehyde treated wood was not darkened. At high humidities,the urea-treated and dried stock absorbed more moisture from the airthan the buffered urea-aldehyde treated and dried stock.

v The treated and end-coated pieces were 7 At humidities above 85percent, the urea-treated stock condensed moisture on its surface,whereas the buffered urea-aldehyde treated stock did not condensemoisture on its surface even at humidities up to 100 percent.

was 2 Paste-piling and kiln drying method Forty pieces of unseasonedwhite oak heartwood 4 by 6 inches in cross-sectional dimensions and 30inches in length, which had been selected dom from a large group of suchpieces, were cover for 12 days after having been surface coated with apaste consisting of the following ingredients:

8 Exam-m3 Soaking and air seasoning method Twenty pieces of unseasonedwhite oak heartwood 3 by 6 inches in cross-sectional dimensions Thesodium hydroxide, borax, and dimethylolurea were dissolved in two-thirdsof the water while heating to 150 F., and the solution was a cooled to80 F. The methylcellulose was wetted in the rest of the water at 180 F.,and this mixture was cooled to 80 F. The urea and the solution were thenadded simultaneously to the methylceilulose-water mixture whilestirring. The bentonite was then added to theentire mixture whilestirring. The paste was applied at the rate of 150 pounds of solidchemical per thousand board feet of the wood.- After the diffusionperiod, the ends of the pieces were treated with a moisture-resistantcoating to prevent moisture loss from the ends and consequent endchecking. piled on I-inch stickers in a drying chamber along with 40similar end-coated pieces which had been treated 12 days with anurea-methylcelluiose paste and 40 similar end-coated pieces which hadnot been given any chemical seasoning treatment. The urea paste wasapplied at the rate of 150 pounds of urea per thousand board feet, andthe stickering and covering conditions were the same as for the bufferedurea-aldehyde paste-treated 'wood during .the diffusion period.

All the pieces were then dried to a moisture content of approximately 12percent using the "drying schedule given in Example 1. The followingtable shows the influence of the buffered ureaaldehyde treatment.

' TABLE 13 Total length in inches Treatment Honeycomb Surface checks pergfig g piwe section Buflered urea-aldehyde, paste-piled. 54. 3 1. 47Urea, paste-piled 138. 3 2. 55 None 153. 4 2. 80

When drying was completed, the color and hygroscopic properties of thebuffered urea-aldehyde paste and urea paste-treated pieces were similarto those of the corresponding pieces treatv ed by those two chemicalseasoning agents by the and 30 inches in length which hadbeen selectedat random from a large group of such pieces were soaked for 20 days in abuffered urea-adlehyde solution prepared as described in Example 1.After the soaking, the ends oi the pieces were coated with amoisture-resistant material to prevent moisture loss from the ends ofthe pieces n consequent end checking. The soakedand l rilliumlulu TABLE0 Total length in inches Bone c mb Treatment Surface checkhig per checksper cross-sect on piece alter kiln drying Buiiered urea-aldehyde,soaked. 61. 4 0.64 Urea, soaked 140. 5 3. 01 None 138. 0 1. 42

When drying was completed, the color and hygroscopic properties of thebuffered urea-aldehyde urea-treated pieces were similar to those of thecorresponding pieces treated by those two chemical seasoning agents andkiln dried.

Exmu 4 Drying under more severe schedules and less exact temperature andhumidity control than permissible with ordinary drying methods to amoisture content of approximately 5 percent using the following dryingschedule:

- Temperature Elapsed Relative 2x 1??? time, w humidity, content DaysDry gulb, etibnlb, Percent Percent 0 140 1 we no 11 no 21 135 25 I 110121 30 131 as The blanks dried without defect, were non-hygro-........i.. ,1 won: darkened only sli htly compared 1 than 60 days,

Although it. is not possible at present to state with certainty whatphenomena are responsible for the high effectiveness of the bufferedureaaldehyde solutions in the chemical seasoning process, it is knownthat the chemicals when impregnated into the wood possess both thehygroscopic and antishrink properties desirable for the process. In theresinified form the absorbed urea-aldehyde improves the bond betweenadjacent and contiguous fibers, thereby increasing the strength of thewood in tension across the grain. This property probably is of someassistance in preventing the formation or enlargement of wood failureswhich appear as checks.

Having thus described my invention, I claim:

1. A process for seasoning refractory wood comprising soaking the woodin an aqueous solution containing urea and formaldehyde in the relativemolal proportions of about 6 to 4, respectively, said solution beingbuffered to a pH in the range of about pH 8 to about pH 10L5, followedby drying the treated wood.

2. A process for seasonin refractory wood comprising soaking the wood inan aqueous solution containing urea and formaldehyde in the relativemolal proportions of about 6 to 4, said solution being buffered to a pHin the range of about pH 8 to about pH 10.5, followed by drying the woodto a moisture content of about 3 to percent, by weight. I

3. A process for seasoning refractory wood comprising applyingurea-formaldehyde resin-forming compounds to the surface of the wood atthe rate of about 40 to 200 pounds per thousand board feet, thecompounds being applied in the form of a solution buffered to a pH inthe range of about pH 8 to pH 10.5 and thickened with a hydrophilicthickening agent, the urea and form- 10 aldehyde being present in therelative molal proportions of about 6 to 4, permitting the resinformingcompounds to diffuse into the wood, followed by dryin the treated wood.

4. As a composition for the chemical seasoning of wood an aqueoussolution of to 60 percent by weight of a mixture of urea andformaldehyde in the relative molal proportions of substantially 6 to 4,respectively, said solution being buffered to a pH in the range ofsubstantially DH 8 to 10.5.

5. As a composition for the chemical seasoning of wood an aqueoussolution of 40 to 60 percent by weight of a mixture of urea andformaldehyde in the relative molal proportions of substantially 6 to 4,respectively, said solution being buflered to a pH inthe range ofsubstantially pH 8 to 10.5 and containing a hydrophilic thickeningagent.

6. As a composition for the chemical seasoning of wood an aqueoussolution of 40 to 60 percent by weight of a mixture of urea andformaldehyde in the relative molal proportions of substantially 6 to 4,respectively, said solution being buffered to a pH in the range ofsubstantially pH 8 to 10.5 and containing starch.

WILLIAM KARL LOUGHBOROUGH.

REFERENCES CITED The following references are of record in the file of'this patent:

UNITED STATES PATENTS Number Name Date 2,019,834. Vierling Nov. 5, 19352,313,953 Loughborough Mar. 16, 1943 2,346,286 Berliner Apr, 11, 19442,376,200 Smidth May 15, 1945 2,398,649 Kvalnes Apr. 16, 1946 OTHERREFERENCES Stamm-Industrial 8: Eng. Chemistry, Oct. 1938,

vol. 28, No. 10. pps. 1164-1169.

Pacific Plastics, vol. 2, No. 14 and I5.

Berliner Plastics (Chicago), July 1944. PPS. 32-34, 90, 91.

1, Jan. 1944, PPS.

